Exposure to the weightless environment of space results in sensorimotor adaptation and physiological de-conditioning with commensurate impacts on astronauts' coordination and abilities to perform physical tasks. The sensorimotor effects are most apparent during critical maneuvering phases of a mission, when physical performance, coordination, and multi-sensory perception are most critical to mission safety and success. Since there are no gravitational “down” cues in space and visual cues may be ambiguous, self-orientation perception with respect to a spacecraft cabin or other weightless environment is constantly changing and may be volitionally commanded. This can lead to difficulty in teleoperation, berthing, or docking tasks, which require the integration of sensory information from multiple reference frames and bimanual coordination. This lack of a common reference direction within the environment or between astronauts may also lead to performance degradation during navigation tasks such as module-to-module locomotion or emergency egress.
Some of the observed sensorimotor effects, such as spatial disorientation and space motion sickness, may be attributed to the initial exposure to weightlessness. Other effects of being in a weightless environment, such as gate ataxia and posture stabilization, have been observed following the transition to a gravitational environment following spaceflight. There currently is no equipment or protocol to facilitate the sensorimotor adaptation from one gravitational environment to another. The sensorimotor effects inhibit astronauts' performance efficacy as they undergo an adaptation period following a transition to weightlessness (following Earth- or partial-G) or a transition back to Earth- or partial-G (following weightlessness).
Exposure to the weightless environment of space also has negative impacts on human health in the long term. In the long term, weightlessness leads to muscle atrophy, muscle strength loss, and skeletal deterioration. To counteract the long term effects, astronauts use time-consuming in-flight exercise regimens to address this loss of muscle strength and bone mass. Compression suits may be worn in an attempt to counteract the physiological de-conditioning, but they are not responsive to their wearer's motions. They do not provide any directional or coordinational movement guidance. Thus, when astronauts engage in physical activities, they have no resistance to undesirable or inappropriate movements. Because the weightless environment of space affects astronauts' motion control and posture stabilization, it can take significantly longer for astronauts to perform physical tasks than it would in an environment with Earth gravity.
Powered exoskeletons for use on land have been developed to augment the strength and endurance of their wearers. However, powered exoskeletons are not intended to provide resistance to movement. Furthermore, powered exoskeletons require a substantial amount of energy for a measured improvement in human strength or endurance.